Prevention of the driven wheels from an excessive slip or spinning on a road surface is extremely useful for the sake of safety as well as for an effective use of the driving force of a vehicle. The prevention may be achieved by decreasing a torque to be transmitted to the driven wheels - a torque being the cause of the slip or spinning.
Slip control systems of this type are disclosed in Japanese Patent Early Publication (Laid-Open) No. 16,948/1983 (corresponding to U.S. Pat. No. 4,484,280) and Japanese Patent Early Publication (Laid-Open) No. 56,662/1985 (corresponding to U.S. Pat. No. 4,583,611). The systems disclosed in these two prior patent applications involve, in each case, a technique that involves the application of a braking force of the brake to the driven wheels and a reduction in the output torque of the engine itself in order to decrease the torque to be transmitted to the driven wheels. More specifically, Japanese Patent Early Publication No. 16,948/1983 discloses a system in which the driven wheels only are braked when a slip of the driven wheels is small, on the one hand, and the output torque of the engine is decreased, in addition to the braking of the driven wheels, when the slip of the driven wheels becomes large, on the other hand. Japanese Patent Early Publication No. 56,662/1985 discloses a system in which, when a slip of only one side of the left and right driven wheels is larger than that of the other side thereof, the one side thereof alone is braked and, when slips of both the left and right driven wheels are large, the both sides of th driven wheels are braked and further the output torque by the engine is caused to be reduced. The prior art systems as disclosed in the above patent applications are such that the braking of the driven wheels by the brake is primarily utilized and the reduction in the output torque of the engine is secondarily utilized.
Japanese Patent Early Publication No. 16,948/1983 further discloses an example of the slip control being conducted against the driven wheel on the outside of the curve at the time of cornering. When the vehicle is being cornered, the load applied to the driven wheel on the inside of the curve is reduced, while the torque applied to the driven wheel on the outside of the curve is increased by the action of a differential gear to cause the spin or spinning of the driven wheel on the outside thereof to increase. Furthermore, it is disclosed that a target value in the slip control is altered so as to decrease when a speed of cornering is faster.
As have been described above, in controlling the slip of the driven wheel to eventually reach a certain target value, how to set such a target value is in issue. In particular, requirements for road holding capabilities of the driven wheel against a road surface may vary with a running state of the vehicle so that some measures are demanded to be taken in this respect.
For the requirements for the road holding capabilities of the driven wheel, it will be necessary to understand gripping forces of the tire on a road surface in the longitudinal direction or in the direction of the propulsion and in the transverse direction or in the widthwise direction of the vehicle body. The gripping force of the tire on the road surface in the longitudinal direction (hereinafter referred to merely as the gripping force unless otherwise stated), on the one hand, gets larger as an amount of the slip or spinning of the driven wheel gets larger and, subsequent to a certain peak point, gets smaller as an amount of the slip or spinning thereof gets larger. The gripping force of the tire on the road surface in the transverse direction (hereinafter referred to merely as the transverse force unless otherwise stated), on the other hand, gets smaller as an amount of the slip or spinning of the driven wheel gets larger. Accordingly, for the slip control, it is desirable to set a target value for the slip or spinning of the driven wheel within a range not exceeding the point where the gripping force becomes maximum in order to secure a sufficient transverse force.
It is, on the one hand, to be noted here that a larger gripping force is preferred in order to secure a propulsion, particularly acceleration, of the vehicle. Thus it is desired to set the target value to a possibly large one. It is, on the other, to be noted here that a larger transverse force is preferred in order to secure a stability of the vehicle. Thus it is desired to set the target value to a possibly smaller one.
The magnitudes of the gripping force and the transverse force vary each with a degree of the slip or spinning of the driven wheel against a road surface, viz., a road surface friction coefficient of the driven wheel even if conditions of driving are the same. Factors affecting the degree of the slip or spinning of the driven wheel include, for example, a state of the surface on a road such as a paved road, an unpaved road, a dry road, a wet road and a frozen road and the kind of a tire such as a normal tire, a snow tire and a spike tire. Even if the degrees of the slip or spinning of the driven wheels would be the same, there are occasions, in accordance with a state of the running of a vehicle, where the transverse force is particularly required, for example, at the time of cornering or where a force of the propulsion is particularly required, for example, at the time of the straight running, particularly at the time of acceleration.
As may have been understood from the foregoing, setting the degree of the slip of the driven wheels to a certain target value is to balance the gripping force with the transverse force at a ratio corresponding to the target value. Accordingly, if the target value is kept at a constant value, it becomes impossible to cope with requirements for different road holding capabilities.
The different road holding capabilities are required, for example, at the time of cornering and the straight running and at the time of the ordinary running and acceleration. Firstly, a large transverse force is required at the time of cornering and a large gripping force is required at the time of the straight running. Second, a large transverse force is required at the time of the ordinary running in order to secure a stability of the vehicle and a large gripping force is required at the time of acceleration.
Another issue is how to set a target value for a degree of the slip of the driven wheel at the start-up time of the slip control, vis., an initial value. If the initial value of the slip control is set to a certain value and thereafter the initial value is altered gradually so as to reach a certain target value for the slip control, it takes a long time to cause the target value to reach an optimum value. For instance, if the initial value is set to a small value as a degree of the slip of the driven wheel is small, a sufficient amount of acceleration cannot be obtained at the initial stage during the slip control at the time of the running on a well-conditioned road. To the contrary, if the initial value is set to a large value with acceleration under consideration, the spinning of the driven wheel is likely to occur from a large amount of the slip at the initial stage when the slip control is conducted at the time of the running on a poorly conditioned road. This results in the problem with stability of the vehicle.